The Supreme Court has declared that Secular Humanism is a religion.
Apologists for theocracy sometimes argue that not only is Secular Humanism a religion, but in fact that the Supreme Court has officially ruled that Secular Humanism is a religion. They then proceed to argue that Secular Humanism is taught in public schools in violation of church/state separation. Since they don't believe that church and state should be separated anyway, their entire point seems to be that public schools should indoctrinate kids into Christianity instead.
Although we don't see nearly as much of it today, even as late as the 1990s there was still quite a lot of attacks against Secular Humanism by the Christian Right. Secular Humanism and Secular Humanists were scapegoats used by the Christian Right to explain all manner of ills throughout American society. Today they have receded to the background, perhaps because of the success of the very vocal "New" atheists and the remarkable progress of equal civil rights for gays.
The attacks once made against Secular Humanists are now targeted at atheists and gays instead, and one popular argument is the one cited above: that something is "really" a religion. Because it's difficult to effectively argue that a system which is explicitly secular and is openly critical of religion is actually a religion itself, it's simpler to just pass off the argument to the Supreme Court. Unfortunately for people making these claims, it is simply not true that the Supreme Court has found Secular Humanism to be a religion.
In the 1961 case of Torcaso v. Watkins, Justice Hugo Black wrote in a footnote that:
Among religions in this country which do not teach what would generally be considered a belief in the existence of God is Buddhism, Taoism, Ethical Culture, Secular Humanism, and others.
As a footnote, this statement qualifies as an obiter dictum — this means that it is simply a personal observation of the judge and hence is only incidental to reaching the opinion. It has no real weight when it comes to legal precedent and cannot be properly considered the "decision" of the court.
Even so, Black was mistaken. If Secular Humanism were a religion, then it wouldn't be secular. The terms "religion" and "secular" are opposites. It is possible that a government body might need to treat secular philosophies like Secular Humanism as if they were religions if not doing so produces unconstitutional results, but otherwise the distinction between Secular Humanism and religion is total.